The conventional method for manufacturing hot rolled galvanized steel sheet is carried out in the following manner. After hot rolling, the hot rolled steel sheet is pickled and then zinc-hot-dip-coated, so that a level of corrosion resistance better than pickled and oiled steel sheet can be obtained, thereby improving the value-added factor. FIG. 1a illustrates a conventional process for manufacturing hot rolled steel sheet with a pickling stop present.
Generally in manufacturing hot rolled steel sheet, scales (referred to as "secondary scales") are formed on the surface of the steel sheet after a rough rolling step during the hot rolling, process. These secondary scales include: a hematite layer as an outermost layer contacting atmospheric air; a magnetite layer just under the hematite layer toward the matrix structure; and a wustite layer closely contacting the matrix structure, the thickness being about 10 .mu.m. These secondary scales greatly deteriorate the coating adherence of the hot rolled galvanized steel sheet, and therefore, the scales are removed by using a pickling solution in which a chloric acid or sulfuric acid solution and a corrosion inhibiting agent are mixed together. However, an oxide layer of about 100-570 .ANG. remains on the surface of the pickled hot rolled steel sheet, and therefore, the zinic coating adherence is significantly aggravated. Therefore, the pickled hot rolled steel sheet is heated to 480-500.degree. C. under an atmosphere of a 7-15%-hydrogen concentration to reduce the oxide layer based on the mechanics of Formulas 1-3 as shown below, and then, a hot dipping is carried out in a zinc bath. EQU 3Fe.sub.2 O.sub.3 +H.sub.2.fwdarw.2Fe.sub.3 O.sub.4 +H.sub.2 O (1) EQU Fe.sub.3 O.sub.4 +H.sub.2.fwdarw.3FeO+H.sub.2 O (2) EQU FeO+H.sub.2.fwdarw.Fe+H.sub.2 O (3)
However, when the scale layer is removed by a pickling, there are generated great differences on a reaction kinetics according to the compositions of the scale layers. Therefore, a part of the matrix structure is over-pickled, and therefore, the surface of the steel sheet becomes rough and irregular, with the result that problems of hydrogen brittleness, iron loss and acid loss can be generated. Further, the pickling has to be completed within a short period of time, and therefore, the operating managements such as the heating condition management, the acid concentration management and the corrosion inhibiting agent concentration management cannot be easily carried out.
Further, a strongly toxic and highly corrosive agent such as a chloric acid solution and a sulfuric acid solution is used. Therefore, a waste acidic solution treating facility have to be installed and maintained, and therefore, the manufacturing cost is increased, while the environmental contamination can become serious.
Further, if the Si content of the steel is 0.1 wt % or more, then the coating adherence is markedly aggravated in the hot rolled galvanized steel sheet. To describe it specifically, if a hot rolled steel sheet contains 0.1 wt % or more of Si which is the coating-fastidious element, there is formed fayalite (2FeO.SiO.sub.2) on the boundary between the scale layer and the matrix structure. This fayalite (2FeO.SiO.sub.2) remains without being removed even after the pickling, thereby forming a non-coated layer. Even if the coating is done, the coating adherence is degraded, thereby inviting later peeling. Thus, a scale layer remains which is not removed by pickling, and the scale layer is not removed even during the subsequent reducing process.
In an attempt to overcome these problems, Japanese Patent Laid-open No. Sho-60-56418 and Hei-5-156416 disclose a method in which the steel sheet is electroplated with Fe, Ni, Cu, Fe--Mn or the like before carrying out the zinc hot dipping. By electroplating the steel sheet, the alloy elements are concentrated on the boundary of the matrix structure when carrying out a high temperature annealing. However, the alloy elements are concentrated under the electroplated layer, and therefore, the alloy elements cannot react with the atmospheric gas, and, therefore, cannot be oxidized. Therefore, in the case where a pickled steel sheet having a rough matrix structure surface is subjected to a hot rolled galvanizing process, a problem occurs in that the amount of coating on the depressed surface is irregular due to the short coating time period. In order to avoid this problem, the electroplating time period is extended or the operation is slowed. However, in this manner, although the non-coating of the depressed portions can be solved, the over-coating of the projected portions cannot be solved. Further, the pre-coated alloy elements have a high hardness and a low ductility, and therefore, if the pre-coated thickness is thick, it will be peeled off later.
In another method as shown in FIG. 1b, a flux treatment is conducted using zinc chloride (ZnCl.sub.2) and ammonium chloride (NH.sub.4 Cl) after completion of the pickling step, thereby providing a discontinuous zinc hot dipping process. In this method, the procedure is complicated, and therefore, the economy is inadequate, as well as being harmful to the environment.
In order to solve the above described problems, methods have been proposed for manufacturing a hot rolled galvanized steel sheet, in which the pickling step is skipped, as shown schematically in FIG. 1c. One example of such is a process disclosed Japanese Patent Laid-open No. Hei-6-145937. In this method, the pickling step is skipped, and the scales are reduced under a reducing atmosphere at a temperature of 300-750.degree. C. This method is effective in solving the above described problems. However, after the hot rolling, the scales consist of 87% magnetite, 6% of wustite and 7% hematite. Therefore, if the magnetite as the major component of the scales is to be reduced, the reduction has to be carried out at a temperature of 650-820.degree. C. for 300 seconds or more. Due to such a long reduction period, the productivity cannot be improved. Further, in this method, if the hot rolled steel sheet is a coating-fastidious steel sheet containing 0.1% or more of Si, a superior coating adherence cannot be ensured like in the other methods which include the pickling step.
Other examples in which the pickling step is skipped, are found in Korean Patent Application Nos. 97-62031 and 97-62032 of the present inventor. In these methods, the temperature and the reducing gas concentration are properly controlled at the reducing and heating zone, and the Al concentration in the zinc bath is optimized, thereby improving the coating adherence. However, in these methods, the scales also contain about 90% magnetite, and therefore, a long time period is required for reducing the magnetite. Thus a fast reduction cannot be expected, and therefore, the productivity cannot be improved.